In the design of electronic circuits, there are several reasons to minimize the number of physical connections made between circuit elements that are remotely located from each other. Examples of such remotely situated elements may be home security intrusion sensors, distributed temperature sensors for commercial kitchens or HVAC sensors, fire alarm networks for office and apartment buildings and many other similar applications. In such applications, three or more wires are necessary between a transmitter and remotely located receiver, one to carry a DC power potential, another to carry a corresponding ground or neutral for the DC power potential and one or more signal wires that carry monitoring or control signals. Connections between such remotely situated elements usually involve the use of electro-mechanical connectors such as plugs, sockets, crimp connectors, pin connectors, automotive connectors and the like. These devices are much less reliable than electronic components themselves, and contribute significantly more to failures than the electronics. In harsh environments, such as automotive and trucking environments, vibration can loosen electrical connectors, and water can eventually find its way into a connector and cause corrosion to the point of opening a circuit made by male and female components of a plug and socket. While more tightly controlled, the same can be said about aircraft connectors. Connectors require physical space, and add to the minimum volume in which a circuit can be mounted. In many modern applications, such as fire alarm and security devices, space is at a premium, and the ability to mount a circuit in a smaller package may significantly increase the perceived value of the product.
Connectors have significant cost in comparison with electronic components, and involve additional conductors, usually made of relatively expensive copper. In some instances, connector terminals may be coated with gold or other expensive, non corrosive conductive element to ward off corrosion. Therefore, reducing the number of conductors in a system usually results in the creation of a smaller, less expensive, faster product that lasts longer and may appear more attractive and more valuable to the customer. These factors combine to cause most designers to seek to minimize the number of conductors and connections in new designs.
The concept of transmitting data over power lines is well established and has been in commercial use for more than 50 years, with many different innovations in method and technique being applied to enable the elimination of one or more conductors and associated connectors. Here, the prior art is based on addition of multiple, sometimes bulky and expensive, components in order to inject data onto the power line in an acceptable and accurate manner.
Earliest methods for employing power lines to carry signals used injection of high frequency signals onto an AC power line. In some instances, digital signals were independent of the AC power and in other instances the signal rides on the AC power sine wave. This method is still in popular usage. In the popular X10 system, digital signals are transmitted during the zero crossing of an AC sine wave. Other methods of signal transmission over power lines involve techniques such as reversing polarity of the power supply and return lines in response to the data to be transmitted. While this method works, it is suited only to a class of applications in which it is possible to reverse polarity in this manner. The method also has significant inefficiencies in power transmission due to losses in the driving and receiving circuits. These circuits are also comparatively bulky and expensive and add to overall heating of the circuit. Dallas Semiconductor has developed a so-called one wire bus that uses a data conductor and ground conductor, and is the subject of their U.S. Pat. No. 6,239,732, issued 1998. The Dallas Semiconductor reference has no power supply, but rather harvests energy from the data line for powering the device. Energy of transmitted bits on the data line is used to charge a capacitor, with the charged capacitor powering their microprocessor. The Dallas Semiconductor system requires a relatively large capacitor for energy storage, in addition to other regulating circuitry. This is in contrast to the instant invention, which uses an existing power source and power line to power both a digital transmitter and receiver, and which can transmit data more efficiently and at much faster data rates. As such, their system is the inverse of the method of the instant application, which modulates a power supply voltage to send data.
In view of the foregoing, it is apparent that there is a need for digital communications that can be implemented over DC power conductors using only two conductors, and which can be implemented using a circuit that costs only a few cents.